A spectrometer is an instrument that quantifies the spectral power density of a polychromatic optical input. Existing spectrometers generally fall into two categories: spectrum splitting spectrometers and Fourier Transform InfraRed (FTIR) spectrometers. A typical spectrum splitting spectrometer spectrally splits the polychromatic input into different channels using a dispersive element (e.g., a diffraction grating, a prism, or a random scattering medium) and then measures the spectral density at each channel. In contrast, an FTIR spectrometer usually employs an interferometer with a variable arm path length to extract the spectral density from an interferogram. Compared to spectrum splitting spectrometers, FTIR spectrometers can offer enhanced signal-to-noise ratio (SNR), since the optical input is not split into multiple channels. This advantage is also referred to as the Fellgett advantage or the multiplex advantage.
Conventional FTIR spectrometers usually include discrete optical elements, such as gratings, prisms, and beam splitters, and therefore can be bulky and costly. These challenges may be addressed by on-chip spectrometers developed by photonic integration technologies. Most on-chip spectrometers are based on spectrum splitting using arrayed waveguide gratings, Echelle gratings, micro-resonators, random scattering medium, or a combination of these dispersive elements. As a result, these on-chip spectrometers usually lack the Fellgett advantage.
Photonic integration technologies can also be used in on-chip FTIR spectrometer, e.g., to realize the arm path length change either by microelectromechanical systems (MEMS) tuning, electro-optic, or thermo-optic tuning. MEMS tuning typically uses mechanical moving parts that can increase complexity of the resulting system and can compromise system robustness. Electro-optic and thermo-optic tuning normally do not use moving parts to change arm path lengths, but the tuning range in these techniques can also be limited, thereby compromising the performance of the resulting spectrometers.
For example, in the near-infrared regime, liquid crystal waveguides can offer a maximum effective index tuning range (e.g., on the order of 10−2). With 10 cm long waveguide, this tuning range of refractive index can provide a spectral resolution of about 10 cm−1. However, the long waveguide length can also increase the device power consumption for tuning. In addition, in the mid-infrared regime (e.g., 2.5 μm<λ<25 μm), liquid crystals can become opaque and electro-optic tuning can provide only a small Δn (e.g., up to 10−3). An alternative to electro-optic tuning can be thermo-optical tuning, but thermos-optical tuning can introduce undesirable blackbody thermal radiation noise. Accordingly, the spectral resolution can deteriorate to an even lower value of about 100 cm−1 for a 10 cm-long interferometer. The apparent trade-off between spectral resolution, device footprint, and power consumption therefore imposes a challenge in developing new spectrometers.